6. Project Summary/Abstract Neurotransmitter:sodium symporters (NSS) couple the accumulation of substrate to the movement of sodium ions down their concentration gradient across the plasma membrane, and as such constitute key elements in cellular signaling and homeostasis. NSS include the transporters for dopamine, serotonin and norepinephrine-targets for amphetamine, cocaine, and antidepressant drugs-as well as the transporters for GABA and glycine, which are targeted for treatment of epilepsy and schizophrenia. In 2005 the Gouaux group solved at 1.65 the structure of LeuT, a bacterial NSS homolog, crystallized with 1 Leu and 2 Na+ bound in an occluded binding pocket (referred to as primary substrate binding (S1) site). The structure provided no easy clues to the pathway of substrate to the S1 site from the extracellular or the intracellular side. An unexpected second substrate binding (S2) site located in the extracellular vestibule was identified during the previous project period; binding and flux experiments showed that the two binding sites can be occupied simultaneously. Substrate in the S2 site allosterically triggers intracellular release of Na+ and substrate from the S1 site, thereby functioning as a symport effector. Because tricyclic antidepressants (TCA) bind differently to this S2 site, they do not promote substrate release from the S1 site and thus act as symport uncouplers to inhibit transport. Identifying the conformational changes associated with transport and the permeation pathways that are formed within the transporter are long term goals of this project critical to understanding the functional mechanisms of the human neurotransmitter transporters and how drugs act upon these mechanisms. To achieve this goal, an integrated approach has been developed based on active collaborations with investigators whose expertise in computational modeling (Harel Weinstein), membrane protein crystallography (Poul Nissen), and single-molecule fluorescence spectroscopy (Scott Blanchard) enables the combined multidisciplinary approach described in this application. The following specific aims are proposed: 1) To use our novel discoveries regarding the specificity and modulation of S2 binding, by detergents, mutations, and ionic substitution, to develop conditions that enable us to understand the regulation of LeuT properties by the S2 binding site and to solve a structure of LeuT with substrate bound to the S2 site. This will provide atomic resolution data to inform our mechanistic hypothesis as to the essential role in transport of substrate binding to this site. 2) To characterize the mechanism of substrate transport in terms of specific conformational changes in the transporter that propagate the allosteric signal triggered by substrate binding to the S2-site towards the intracellular gate of the transporter and allow inward release of substrate. 3) To establish the relevance of our structural and functional findings in bacterial transporters to understanding the function of SERT and DAT. We will: a) demonstrate the essential functional role of the S2 site in these human transporters, and b) use a Cl-- dependent LeuT mutant to determine the structure of the Cl- binding site and thus to explicate the functional role of Cl- in SERT and DAT.